Generally these days I'm in the camp that the OS no longer matters at all, as long as it has a good web browser.
However I have an XP box and a linux box at work, and a mac at home. The mac is over 2 years old, and the XP machine is only ~9 months old. The mac has no problems....but the bitrot on the XP box is so awful right now that I can barely use it. I write video encoding software, and performance tests on the Windows machine are running about half the speed now than they were running 6 months ago....even though windows claims nothing is running.
I don't have time to wipe the XP machine, and I need it to do my job. If you don't mind reinstalling every 6 months, Windows does just fine, but I'm pretty unhappy right now that I have to spend an entire day just to get back up to a productivity level I used to be at.